What is a Conjugate Acid?
A conjugate acid, as defined by the Bronsted-Lowry theory of acids and bases, is the species that accepts a hydrogen proton during a chemical reaction. This means that the conjugate acid is the base reactant after it has taken the hydrogen proton from the original acid.
There is generally some confusion as to why a conjugate acid accepts a proton while the original acid in the equation donates a proton. This is simply because the exchange between these two species allows the conjugate acid to become an acid, by definition, for the next reaction. Because the conjugate acid accepts a hydrogen ion, it is now able to donate that ion in a new reaction – just like the original acid would have. Additionally, the equation can easily be reversed and maintain the same outcome.
A Brief History of Acids and Conjugate Acids
Although acids have been an integral part of chemistry and related disciplines since the first studies of elements and their properties had been done, there was no proper definition for what constituted an acid until 1884 when Svante Arrhenius came up with the first definitions for acids and bases.
Although this definition was great progress in the field of chemistry, it was limited in that it could only determine an acid or a base by examining how the element dissociated in water. Arrhenius determined that when an acid comes into contact with water, it releases positive hydrogen ions. Additionally, Arrhenius determined that when a base comes into contact with water, it creates negative hydroxide ions.
This definition served as the primary identification of acids and bases until 1923 when two scientists discovered another way to categorize acids and bases separately. After years of study, Johannes Nicolaus Bronsted and Thomas Martin Lowry both discovered another way to determine acids and bases – even in reactions that didn’t involve water. They realized that in any given reaction, an acid would donate a hydrogen ion and a base would accept the hydrogen ion. This realization soon gave way to more discoveries concerning the products – conjugate acids and bases.
How are Conjugate Acids and Bases Determined?
The Bronsted-Lowry definition of acids and bases says that the acids and bases can be determined by examining which of the reactants donates a proton and which of the reactants accepts a proton. The acid (positively charged) will always donate a proton while the base (negatively charged) will always accept the proton, as seen below with the dissociation of ammonium.
Ammonium ions are weaker acids. When they donate a proton to a different element or species, the above reaction shows how the conjugate acid and base are created. The H+ ion accepted a proton from NH4, making it the conjugate acid. Therefore, the NH3 ion is now a weak base (the conjugate base).